WE HAVE SEVERAL REPORTS BEGINNING WITH NBC'S RON ALLEN IN SEOUL, SOUTH KOREA >> Reporter: AN EXTRAORDINARY ACT OF DEFIANCE

NORTH KOREA RELEASING PICTURES OF KIM JONG-UN INSPECTING WHAT IT CLAIMS IS A HYDROGEN BOMB THAT COULD BE MOUNTED AND LAUNCHED BY AN INTERCONTINENTAL BALLISTIC MISSILE THEN, HOURS LATER, A MASSIVE UNDERGROUND BLAST DETECTED AT A NUCLEAR TESTING SITE, MEASURED AT 63 MAGNITUDE BY THE US GEOLOGICAL SURVEY, FOLLOWED BY A SMALLER BLAST MINUTES LATER, A COLLAPSE

NORTH KOREA STATE MEDIA ANNOUNCING THE COUNTRY'S SIXTH NUCLEAR TEST, A PERFECT SUCCESS THE REGIME'S MOST POWERFUL TEST YET, ESTIMATED AT SOME FOUR TO FIVE TIMES GREATER THAN THE NUCLEAR BOMB THE US DROPPED ON HIROSHIMA DURING WORLD WAR II TREMORS FELT IN SOUTH KOREA

BUILDINGS REPORTEDLY SHAKING IN CHINA AND RUSSIA A SENIOR US OFFICIAL SAYS THE ANALYSIS SO FAR IS CONSISTENT WITH NORTH KOREA'S CLAIM IT TESTS AN ADVANCED NUCLEAR DEVICE SEEN IN THE REGION AS A DIRECT THREAT TO THE UNITED STATES

>> I DON'T THINK THE NORTH KOREANS ARE AFRAID OF DONALD TRUMP RIGHT NOW I THINK THEY SEE A PRESIDENT WITH A LOW APPROVAL RATING, A LOT OF DOMESTIC DISCORD I THINK TO SOME EXTENT, THEY MAY BE TAKING ADVANTAGE OF THAT AND KIND OF PUSHING THE ENVELOPE >> Reporter: THE NORTH KOREAN REGIME RAPIDLY EXPANDING ITS NUCLEAR AND MISSILE CAPABILITY UNDER THE SO-CALLED SUPREME LEADER, THREATENING TO ATTACK GUAM, FIRING A LONG-RANGE MISSILE OVER JAPAN TODAY, SOUTH KOREA CALLED FOR THE STRONGEST POSSIBLE MEASURES AGAINST THE NORTH WHILE WARNING WAR WOULD BE CATASTROPHIC

AND JAPAN ASKED FOR AND RECEIVED REASSURANCES OF US MILITARY PROTECTION DEFENSE SECRETARY JAMES MATTIS CALLED THE US

COMMITMENT TO ALLIES IRONCLAD >> WE ARE NOT LOOKING TO THE TOTAL ANNIHILATION OF A COUNTRY, NAMELY NORTH KOREA BUT AS I SAID, WE HAVE MANY OPTIONS TO DO SO THANK YOU VERY MUCH >> Reporter: FROM AROUND THE WORLD, THERE'S BEEN CONDEMNATION OF NORTH KOREA AND CALLS FOR THE UNITED STATES AND ITS ALLIES TO NEGOTIATE A SOLUTION TO THIS CRISIS